The present invention relates to an apparatus and method for cleaning a semiconductor substrate.
Recently, demands for exceptionally fine wiring patterns in semiconductor devices have become the norm. Not only are there increasing demands being made for device downsizing, but also for improved reliability in such devices. As the distance between wiring patterns decreases, it becomes increasingly important to avoid contamination of substrate surfaces with particulate and other contaminants, in order to prevent the occurrence of short circuits and other defects. Consequently, cleaning of semiconductor substrates is now required to be carried out at various steps in semiconductor manufacturing processes.
In this connection, a cleaning technique presently employed in CMP (Chemical/Mechanical Polishing) will be described below by way of example.
In CMP, an abrasive such an Al2O3, SiO2, and CeOx in a slurry or polishing liquid adheres to a wafer surface after polishing. In the case of a silicon wafer with a diameter of 200 mm, about 4 to 4xc3x97104 particles of 0.2 microns in diameter adhere to the wafer surface. The wafer surface in this state is subjected to both primary and secondary cleaning processes as described below.
In the primary cleaning process, the wafer is held by a plurality of driving rollers which are engaged with the peripheral edge of the wafer, and the driving rollers are rotated to cause the wafer to be rotated about an axis. Sponge rollers are then pressed against opposite sides or obverse and reverse sides of the rotating wafer so as to remove from its surface any particles including abrasive particles and debris which have become detached from the wafer. However, in the case of a wafer having recesses formed on its surfaces, sponge rollers are unable to be brought into adequate contact with the wafer surface due to the existence of the recesses.
The secondary cleaning process will be described below with reference to FIG. 21. FIG. 21 is a conceptual view of a cleaning apparatus used in the cleaning process. The silicon wafer 1, which has already been subjected to the primary cleaning process, is held by a plurality of driving rollers (not shown) which are engaged with the peripheral edge of the wafer. Rotation of the driving rollers causes the wafer 1 to rotate about an axis in the direction of the arrow. An ultrasonic nozzle 31 is provided above the surface of the wafer 1. The ultrasonic nozzle 31 is operated to move in a diametrical direction of the wafer. A cleaning liquid 33 is applied to the wafer 1 from the nozzle 31 to remove any particles remaining on the surface of the wafer 1. It should be noted that in the secondary cleaning process, ultrasonic vibrations are imparted to the cleaning liquid 33 by an ultrasonic vibrator incorporated in the nozzle 31 in order to effect propagation of vibrations to the surface of the wafer 1 through the cleaning liquid 33. The application of ultrasonic vibrations to the wafer 1 enables cleaning to be enhanced greatly as a result of a synergistic effect obtained by a combination of a chemical cleaning effect provided by the cleaning liquid and a direct physical cleaning effect induced by ultrasonic vibrations imparted to the wafer under cleaning.
However, the cleaning apparatus shown in FIG. 21 involves a problem in that a relatively long amount of time is required in order to complete a cleaning process. FIG. 22 is a conceptual view of a cleaning apparatus comprising an elongated nozzle 41, which was devised to shorten a required cleaning time. As shown in FIG. 22, a wafer 1 is held and rotated by driving rollers engaged with the peripheral edge thereof. The elongated nozzle 41 is placed above the wafer to extend in a diametrical direction of the wafer 1. The elongated nozzle supplies a cleaning liquid imparted under ultrasonic vibrations along the entire length of the diameter of the water, whereby the cleaning time can be shortened in comparison to a cleaning apparatus as shown in FIG. 21.
However, this highly efficient cleaning effect obtained by the cleaning apparatus shown in FIGS. 21 and 22, is performed only with respect to the obverse side of a wafer facing the nozzle of the cleaning apparatus, and the cleaning effect obtained at the reverse side is inferior to that obtained at the obverse side. Although adoption of a cleaning process whereby the wafer is turned around to effect cleaning of the reverse side is conceivable, such a process would double the amount of time required to clean a wafer. There has also been proposed a cleaning method wherein a wafer is dipped in its entirety into a cleaning liquid with ultrasonic waves being imparted to the wafer once it is immersed in the cleaning liquid. However, this method is subject to a problem in that it causes an uneconomic increase in the amount of chemical liquid used. In addition, particles, including abrasive particles removed from the wafer, tend to adhere to the wall of a vessel in which the cleaning liquid is contained. Adhesion of such particles has the potential to cause adverse effects during cleaning of a wafer.
The object of the present invention is to overcome the problems described in the foregoing passages. In particular, these problems include a highly efficient cleaning effect that can be obtained only with respect to a wafer surface facing an ultrasonic vibrator, and a cleaning effect obtained at the reverse side that is inferior to that obtained at the obverse side.
In view of the above-described problems with the prior art, an object of the present invention is to provide a semiconductor substrate cleaning apparatus and method capable of efficiently removing contamination from both the obverse and reverse sides of a semiconductor substrate.
The present invention provides a semiconductor substrate cleaning apparatus including a cleaning liquid supply nozzle for supplying a cleaning liquid to both the obverse and reverse sides of a semiconductor substrate to be cleaned. The cleaning apparatus further includes an ultrasonic vibrator for applying ultrasonic waves to both the obverse and reverse sides of the semiconductor substrate.
Preferably, the ultrasonic vibrator is placed in contact with the semiconductor substrate to apply vibrations directly to the semiconductor substrate. Alternatively, the ultrasonic vibrator is placed at a distance from the semiconductor substrate to apply vibrations to the semiconductor substrate through the cleaning liquid or a protective member disposed between the ultrasonic vibrator and the semiconductor substrate.
Preferably, the cleaning apparatus is provided with a plurality of retaining jigs placed in contact with the outer peripheral edge of the semiconductor substrate. The retaining jigs are adapted to rotate while being pressed against the outer peripheral edge of the semiconductor substrate, thereby retaining and rotating the semiconductor substrate. More desirably, the retaining jigs each incorporate the ultrasonic vibrator.
Preferably, the cleaning apparatus has sponge rollers provided for both the obverse and reverse sides of the semiconductor substrate. The sponge rollers are adapted to rotate in contact with the semiconductor substrate, thereby removing contamination from both the obverse and reverse sides of the semiconductor substrate. Preferably, the vibration frequency of the ultrasonic vibrator is in the range of from 200 kHz to 700 kHz. The most suitable vibration frequency of the ultrasonic vibrator is in the range of from 400 kHz to 500 kHz.
It should be noted that the cleaning apparatus may have a single ultrasonic vibrator or a plurality of ultrasonic vibrators and a single cleaning liquid supply nozzle or a plurality of cleaning liquid supply nozzles. However, preferably a single vibrator and a single nozzle are employed in such a manner that the vibrator is incorporated in the nozzle. In this case, it is preferable that the cleaning liquid is discharged from the nozzle as a jet towards the peripheral edge of the semiconductor substrate with an angle in a range from xc2x110 to 20xc2x0 with respect to the surface of the semiconductor substrate. In a case where a plurality of ultrasonic vibrators are provided, the ultrasonic vibrators are provided in symmetry with respect to the surface of the semiconductor substrate, and ultrasonic vibrations having the same characteristics are imparted to the semiconductor substrate at the same angle and in symmetry between the obverse and reverse sides of the semiconductor substrate.
It is desirable that the pH of the cleaning liquid be not less than 7.
In addition, the present invention provides a semiconductor substrate cleaning method wherein a cleaning liquid is supplied simultaneously to both the obverse and reverse sides of a semiconductor substrate to be cleaned, and ultrasonic waves are imparted to both the obverse and reverse sides of the semiconductor substrate, thereby cleaning the semiconductor substrate.
In the present invention, a cleaning liquid having ultrasonic vibrations is simultaneously supplied from the cleaning liquid supply nozzle to both the obverse and reverse sides of the semiconductor substrate. Accordingly, the obverse and reverse sides of the semiconductor substrate can be cleaned simultaneously. Therefore, the cleaning time can be shortened. Because the cleaning liquid is supplied from the nozzle, the amount of chemical liquid used is reduced even in comparison to the dipping type cleaning process in which the whole semiconductor substrate is dipped in the cleaning liquid.
By providing each retaining jig with an ultrasonic vibrator for imparting ultrasonic vibrations to the cleaning liquid, ultrasonic vibrations can be applied simultaneously to the obverse and reverse sides of the semiconductor substrate.
By adopting a structure in which the ultrasonic vibrators provided in the driving rollers are placed in direct contact with the semiconductor substrate, ultrasonic vibrations can be applied directly to the semiconductor substrate without using the cleaning liquid as a vibration medium. Thus, ultrasonic vibrations can be continuously applied in the diametrical direction of the semiconductor substrate by shock waves passing through the semiconductor substrate.
In a case where a single ultrasonic vibrator and a single cleaning liquid supply nozzle are intergrated into one unit, ultrasonic vibrations can be imparted to a side of the semiconductor substrate from the nozzle tip. Therefore, it is possible to clean both the obverse and verse sides of the semiconductor substrate simultaneously and to minimize the costs of the
By providing the cleaning apparatus with sponge rollers for cleaning, it becomes to clean the semiconductor substrate by a single cleaning process in contrast to the conventional system that requires two steps for cleaning. Accordingly, the cleaning time can be shortened, and the cleaning effect is improved remarkably.